There have been known vehicle control systems having an engine and a hydro-mechanical transmission (HMT) which transmits engine power from an input shaft to an output shaft through a mechanical transmission unit and a hydrostatic transmission unit (e.g., Japanese Patent Kokoku Publication No. 62-31660). Since mechanical power can be transmitted with high efficiency, the transmission (HMT) provided for this vehicle control system is designed to convert only part of mechanical power into hydraulic power, so that it can achieve high transmission efficiency. Such a transmission is considered to be an ideal transmission for vehicles subjected to wide load variations such as bulldozers and wheel loaders and is, in fact, employed in some vehicles.
In a typical hydro-mechanical transmission (HMT), variable speed characteristics are achieved by a planetary gear train. More concretely, the transmission is arranged as follows. Of three elements (i.e., the sun gear, the carrier provided with planetary gears, and the ring gear) of the planetary gear train, a first element is coupled to the input shaft, a second element is coupled to the output shaft, and a third element is coupled to a hydraulic pump or hydraulic motor. The rotational speed of the hydraulic pump or hydraulic motor is varied thereby changing the rotational speed of the output shaft.
In the known art, there are basically two types of HMTs. One is the “output-split type” where a hydraulic pump or hydraulic motor, which is connected to another hydraulic pump or hydraulic motor coupled to the planetary gear train by means of a hydraulic circuit, is coupled to the input shaft of the transmission so as to obtain a constant speed ratio. The other is the “input-split type” where a hydraulic pump or hydraulic motor, which is connected to another hydraulic pump or hydraulic motor coupled to the planetary gear train by means of a hydraulic circuit, is coupled to the output shaft of the transmission so as to obtain a constant speed ratio. Further, the output-split type and input-split type are respectively classified into six types according to which of the three elements of the planetary gear train is coupled to the hydraulic pump, hydraulic motor or input/output shafts and, in total, 12 types are available as basic combinations.
The conventional output-split type HMT and input-split type HMT will be respectively described in more detail.
FIG. 12(a) shows a schematic structural diagram of an output-split type HMT. In this output-split type HMT 100, a first gear 103 is secured to an input shaft 102 to which power from an engine 101 is input. A second gear 104 meshing with the first gear 103 is secured to a shaft 105a of a first pump/motor 105. Secured to the input shaft 102 is a sun gear 107 of a planetary gear train 106. A plurality of planetary gears 108 are disposed so as to mesh with the periphery of the sun gear 107. Each planetary gear 108 is axially supported by a planetary carrier 109 to which an output shaft 110 is secured. A ring gear 111 meshes with the periphery of the planetary gear set 108. Meshing with the periphery of the ring gear 111 is a third gear 112 which is, in turn, secured to a shaft 113a of a second pump/motor 113. In this arrangement, the first pump/motor 105 is hydraulically connected to the second pump/motor 113 by a piping 114.
In such a system, when the rotational speed of the second pump/motor 113, that is, the rotational speed of the ring gear 111 is zero, hydraulically transmitted power becomes zero so that all power is transmitted through the mechanical unit. On the basis of the rotational speed of the output shaft 110 at that time, the operation of this system will be described below.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power through the medium of hydraulic pressure and is then activated to increase the rotational speed of the output shaft 110. At that time, the first pump/motor 105 serves as a pump whereas the second pump/motor 113 serves as a motor, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted in the form of hydraulic power becomes plus (+) as indicated by line A-B in FIG. 12(b) and the hydraulic power flows in a forward direction, i.e., from the input shaft 102 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted in the form of hydraulic power becomes minus (−) as indicated by line A-C in FIG. 12(b) and the hydraulic power flows in a reverse direction, i.e., from the planetary gear train 106 toward the input shaft 102.
FIG. 13(a) shows an input-split type HMT 200 in which the planetary gear train 106 is disposed on the side of the input shaft 102 whereas the first pump/motor 105 is disposed on the side of the output shaft 110. In FIG. 13(a), the parts that are substantially equivalent or function substantially similarly to those of the transmission 100 shown in FIG. 12(a) are indicated by the same numerals as in FIG. 12(a), and a detailed explanation of them is skipped herein.
The input-split type transmission 200 is constructed as follows.
(1) When increasing the rotational speed of the output shaft 110, the second pump/motor 113 serves as a motor while the first pump/motor 105 serves as a pump, so that energy is transmitted from the first pump/motor 105 to the second pump/motor 113 through the medium of hydraulic pressure. Then, the horsepower transmitted in the form of hydraulic power becomes minus (−) as indicated by line A-D in FIG. 13(b) and the hydraulic power flows in a reverse direction, i.e., from the output shaft 110 toward the planetary gear train 106.
(2) When reducing the rotational speed of the output shaft 110, the second pump/motor 113 receives motive power from the planetary gear train 106 and rotates in a direction opposite to that of the case (1). At that time, the second pump/motor 113 serves as a pump whereas the first pump/motor 105 serves as a motor, so that energy is transmitted from the second pump/motor 113 to the first pump/motor 105 through the medium of hydraulic pressure. Then, the horsepower transmitted in the form of hydraulic power becomes plus (+) as indicated by line A-E in FIG. 13(b) and the hydraulic power flows in a forward direction, i.e., from the planetary gear train 106 toward the output shaft 110.
As such, in both of the output-split type and input-split type transmissions, energy flows in forward and reverse directions occur in the speed increasing area and the speed reducing area. The energy transmission efficiency in this case will be hereinafter examined, taking the output-split type HMT 100 shown in FIG. 12 for example. Herein, the transmission efficiency of the mechanical unit is 95% and the transmission efficiency of the hydrostatic unit is 80% (Generally, where pump-motors are used, transmission efficiency is low). For easy comparison, assume that the amount of engine power is 1.0 and one third the engine power is input to the hydrostatic unit.
FIG. 14(a) shows the case where hydraulic power flows in the forward direction. Specifically, one third (0.333 part) the energy output from the engine 101 flows to the hydrostatic unit for increasing speed. Transmitted to the output shaft 110 are 0.633 (=(1−⅓)×0.95) part of energy from the mechanical unit and 0.267 (=0.333×0.8) part of energy from the hydrostatic unit. As a result, the overall efficiency becomes 0.9 (=0.633+0.267). The case where hydraulic power flows in the reverse direction is shown in FIG. 14(b). In this case, 1.267 (=1+0.267) parts of energy are input to the mechanical unit and 1.20 (=1.267×0.95) parts of energy are transmitted, so that the overall efficiency is 0.870 (=1.20−0.333).
As just described, when hydraulic power flows in the reverse direction, a large flow of energy occurs in each element, resulting in poor efficiency. In other words, a forward flow of hydraulic energy is better than a reverse flow of hydraulic energy. As seen from FIGS. 14(a) and 14(b), if part of energy flows in the reverse direction, the amount of energy that passes through the mechanical unit will increase, and therefore, there arises a need to increase the size of the planetary gear train, which leads to a disadvantage in economical efficiency.
As an attempt to solve the problems of the prior art output-split type HMT and input-split type HMT, there has been proposed a transmission capable of serving as an output-split type HMT when the rotational speed of the output shaft is increased and as an input-split type HMT when the rotational speed of the output shaft is reduced (Hereinafter, this proposed transmission is referred to as “output-split/input-split switching type HMT”). The output-split/input-split switching type HMT has several advantages. For instance, the horsepower transmitted in the form of hydraulic power can be kept to be plus irrespective of the rotational speed of the output shaft, so that hydraulic power can be allowed to constantly flow in the forward direction and increased energy efficiency can be achieved in all speed regions ranging from the low speed region to the high speed region.
A vehicle control system having an engine and the output-split/input-split switching type HMT described above is constructed as follows. Gear shifting is carried out such that, as shown in FIG. 15, an engine output torque value TQ corresponding to an engine speed NQ in a lower speed region is set as an input torque value and the output-split/input-split switching type HMT generates, from its output shaft, output torque which matches tractive force required by a load, while keeping the input torque value constant. Then, a tractive force-vehicle speed characteristic line (represented by chain line WLQ in FIG. 15) in the lower speed region is set based on the gear shifting operation described above. Also, gear shifting is carried out such that, an engine output torque value TH corresponding to an engine speed NH in a higher speed region is set as an input torque value and the output-split/input-split switching type HMT generates, from its output shaft, output torque which matches tractive force required by a load, while keeping the input torque value constant. Then, a tractive force-vehicle speed characteristic line (represented by chain line WLH in FIG. 15) in the higher speed region is set based on the gear shifting operation described above. Further, a direct region, which is an engine speed region between the engine speed NQ in the lower speed region and the engine speed NH in the higher speed region and in which power transmission from the input shaft to the output shaft is carried out by the mechanical transmission unit alone in the output-split/input-split switching type HMT, is established in a frequently used area in the characteristic graph showing the relationship between tractive force and vehicle speed, and a tractive force-vehicle speed characteristic line (represented by solid line WLG in FIG. 15) corresponding to the direct region is set.
In addition, the tractive force-vehicle speed characteristic line WLQ in the lower speed region is designed to be used for an input-split type HMT, whereas the tractive force-vehicle speed characteristic line WLH in the higher speed region is designed to be used for an output-split type HMT. “The all-speed control” for controlling engine speed in all speed regions as indicated by regulation lines RL0 to RL6 in FIG. 15 is employed for engine control, because in the all-speed control, engine speed fluctuations due to variations in the load are less likely to occur and, therefore, high stability can be ensured. It should be noted that, in FIG. 15, the engine speed NQ in the lower speed region is an engine speed corresponding to the maximum torque point of the engine, whereas the engine speed NH in the higher speed region is an engine speed corresponding to the rated torque point (i.e., the output torque point at which the output of the engine becomes a rated output) of the engine. The point, which is specified by the engine speed NQ in the lower speed region associated with the setting of the tractive force-vehicle speed characteristic line WLQ in the lower speed region and by the engine output torque value TQ corresponding to the engine speed NQ in the lower speed region, is hereinafter referred to as “a matching point MQ in the lower speed region”. The point, which is specified by the engine speed NH in the higher speed region associated with the setting of the tractive force-vehicle speed characteristic line WLH in the higher speed region and by the engine output torque value TH corresponding to the engine speed NH in a higher speed region, is hereinafter referred to as “a matching point MH in the higher speed region”.
The vehicle control system having the output-split/input-split switching type HMT, however, suffers from the following problem, owing to the facts that the matching point MQ in the lower speed region and the matching point MH in the higher speed region need to be spaced at a certain distance in order to effectively form the direct region and that the all-speed control is employed for engine control. Specifically, where a balancing point Q between a tractive force FQ required by a load and a vehicle speed VQ lies on the tractive force-vehicle speed characteristic line WLQ, the engine conforms to the output-split/input-split switching type HMT at the matching point MQ in the lower speed region, even if the opening of the engine throttle is reduced in conjunction with decelerating operation of the decelerator or the like such that the present regulation line is shifted from the regulation line RL0 for full throttling to the regulation line RL4 which passes through the matching point MQ in the lower speed region, by way of the regulation lines RL1, RL2 and RL3. Therefore, the output speed of the output-split/input-split switching type HMT cannot be changed by the shifting of the regulation line. In short, this vehicle control system has the problem that there exists a vehicle speed region where vehicle speed cannot rapidly decrease even when the opening of the engine throttle is reduced in conjunction with deceleration.
The invention is directed to overcoming this problem and a primary object of the invention is therefore to provide a vehicle control system capable of rapidly carrying out deceleration in all vehicle speed regions and setting a direct region where power from the engine is transmitted through the mechanical transmission unit alone in the hydro-mechanical transmission.